Use of polymeric materials is gaining increased acceptance in the biomedical industry. Many polymeric parts for biomedical products (referred to herein as “biopolymers”) are produced in small and medium-sized batches. Accordingly, due to the costs associated with tooling, machining of biopolymers represents a cost-effective alternative to molding or extruding. Unfortunately, the physical properties of some polymeric materials, tight tolerances required in many biomedical products, and process limitations associated with biopolymers (which will be imbedded in the human body) present challenges for conventional machining processes.
The machinability of polymeric materials depends primarily on material characteristics (glass transition temperature [Tg], melt temperature [Tm], molecular weight and viscosity), as well as machining process conditions (cutting speed, cutting edge radius, tool angles and tool surface tribological properties). The stiffness of most polymers is highly dependent upon temperature. As polymers are cooled through and below their Tg, their stiffness increases dramatically, typically by several orders of magnitude.
For polymers having a Tg near or below room temperature (e.g. acrylic-based hydrophobic copolymer, with a Tg of 5 degrees C. to 20 degrees C.), it is very difficult to produce a smooth, uniform machined surface if the polymer is machined at typical room temperature/indoor ambient temperatures (e.g., between 20 and 30 degrees C.). Improved cooling techniques are also need for polymers with Tg that are well above room temperature, such as polymethyl methacrylate (PMMA), having a Tg of 110 degrees C. to 120 degrees C., or polyetheretherketone PEEK, having a Tg of 138 degrees to 149 degrees C. Machining such polymers without adequate cooling can cause the material to smear, as the machining temperature gets above glass transition temperature. This can also cause increased burr formation, localized melting and surface waviness.
Efforts have been made to increase stiffness of polymers by lowering the temperature of the polymer using “ice-blocking,” cold air guns and cryogenic cooling. Each of these cooling approaches has serious deficiencies that prevent them from being effective. Particularly for polymers with glass transition temperatures well below room temperature (e.g. silicone, with a Tg of −90° C. to −120° C.), ice-blocking and cold air guns cannot cool the polymer to a sufficiently low temperature. For example, machining of the acrylic-based hydrophobic blanks at room temperature with compressed air cooling results in significant tearing of the material, resulting in unacceptable surface finish (see FIG. 2A). In addition, any cooling technique that introduces moisture on a hydrophilic polymer and/or leaves a residue on a biopolymer will also be problematic.
Efforts have been made to use conventional cryogenic cooling techniques to cool polymers during a machining process. Such efforts have proven problematic due to, in part, the inability of conventional cryogenic cooling systems to provide a controlled cooling at temperatures well-above the vaporization temperature of the cryogenic fluid. Therefore, jetting of a cryogen on the part rapidly reduces part temperature to well below Tg, which can cause part cracking or brittle fracture during machining.
Dry machining of polymers, such as PEEK, can generate a significant amount of burrs. Current industry practices to remove burrs include time-consuming manual removal by brushing and jetting of abrasive solutions. Brushing at room temperature is often not effective because the machining process often leaves the burrs soft and pliable, due to their small mass. Jetting of abrasive solutions is problematic because it can cause removal of micro-features and/or melting of the material. Conventional cryogenic cooling techniques suffer from the same overcooling problems associated with cryogenic cooling during machining. Therefore, there is a need for an improved process for burr and flashing removal for machined polymers.